Recovery of niobium and tantalum

ABSTRACT

NIOBIUM AND TANTALUM ARE RECOVERED AS PHOSPHIDES FROM A CRUDE OXIDE OR MINERAL FEED MATERIAL BY SELECTIVE ELECTRODEPOSITION FROM A FSED SALT BATH COMPRISING THE FEED MATERIAL, AN ALKALI METAL PHOSPHATE, AN ALKALI METAL HALIDE AND AN ALKALI METAL BORATE OR BORIC OXIDE, ELECTROLYSIS AT A POTENTIAL OF 2.5 VOLTS OR LESS RESULTS IN DEPOSITION OF NIOBIUM PHOSPHIDE ESSENTIALLY FREE OF TANTALUM, WITH SUBSEQUENT ELECTROLYSIS AT 4.0 VOLTS OR GREATER RESULTING IN DESPOSITION OF TANTALUM PHOSPHIDE.

United States Patent 3,725,221 RECOVERY OF NIOBIUM AND TAN TALUM John M. Games, 1650 Rayburn Drive 89503; Kenji Uchida, 1341 Hillside Drive 89502; and Morton M. Wong, 2281 Riviera St. 89502, all of Reno, Nev. No Drawing. Filed Jan. 26, 1972, Ser. No. 221,092

Int. Cl. B01k 1/00 US. Cl. 204-61 Claims ABSTRACT OF THE DISCLOSURE Niobium and tantalum metals have excellent high temperature properties and are resistant to attack by liquid metals and acids. Both metals form important alloys with ferrous metals and other refractory metals, the alloys having numerous space, electronic and defense applications. In addition, the phosphides find utility as corrosion resistant coatings, semi-conductors and hard metal compounds.

Conventional procedures for recovery of pure niobium and tantalum from their minerals require a complex sequence of processing operations. This sequence involves alkali fusion to decompose the mineral concentrate, followed by hydrofluoric acid leaching. A11 intermediate impure niobium-tantalum oxide or fluoride is obtained, which is then purified by wet chemical methods to obtain a pure niobium-tantalum compound. Separation of niobium and tantalum is achieved by a liquid-liquid extraction technique with an SO-percent recovery of pure oxides of the two elements being obtained. The oxides are then reduced to metal by metallothermic or electrolytic techniques.

It has now been found, according to the process of the invention, that niobium and tantalum may be recovered from their minerals by means of a much simpler and more eflicient process in which the two metals are selectively electrolytically deposited as monophosphides from a fused salt bath comprising alkali metal phosphate, halide and borate or boric oxide. Selective electrodeposition is achieved by initial deposition of NbP at a potential of 2.5 volts or less, followed by deposition of TaP at a potential of 3.0 volts or greater.

The feed material in the process of the invention is essentially a mixed oxide of niobium and tantalum, but it may also contain oxides of other elements such as iron, manganese, sodium, titanium, silicon, etc., depending on the composition of the mineral from which it is derived. It may be derived from suitable minerals by the conventional alkali fusion and acid leaching described above, or it may consist of a concentrate obtained by bcneficiating a niobium and tantalum-containing mineral. Examples of such minerals are columbite, pyrochlor and euxenite. The preferred feed material consists of a columbite concentrate containing about 50 to 60 wt. percent Nb 0 5 to 25 wt. percent Ta O to wt. percent Fe O and 2 to 10' wt. percent MnO.

The feed material is initially dissolved in a molten electrolyte comprising (1) an alkali metal phosphate, (2) an alkali metal halide and (3) an alkali metal borate or boric oxide. Examples of suitable alkali metal phosphates are sodium pyrophosphate, sodium metaphosphate, potassium pyrophosphate, potassium metaphosphate, lithium pyrophosphate and lithium metaphosphate. Suitable alkali metal halides comprise sodium chloride, sodium fluoride, cryolite (Na AlF potassium chloride, potassium fluoride, potassium aluminum tetrafluoride (KAIF lithium chloride and lithium fluoride. A combination of sodium chloride and sodium fluoride has been found to give particularly good results. Suitable alkali metal borates include sodium tetraborate, potassium tetraborate and lithium tetraborate.

Optimum proportions of feed material and components of the electrolyte will vary considerably, depending on the specific materials employed, and are best determined experimentally. However, the following proportions, in percent by weight of the molten bath, including the feed, are generally satisfactory: feed material, 4 to 12 percent; alkali metal phosphate, 9 to 25 percent; alkali metal halides, 60 to percent; and alkali metal borate or boric oxide, 8 to 15 percent. It has also been found that further addition of cryolite and potassium carbonate to the electrolyte after deposition of NbP improve the subsequent recovery of TaP. Amounts of cryolite and potassium carbonate for this purpose are suitably about 15 to 20 weight percent and 2 to 5 weight percent, respectively.

The operating temperature, i.e., the temperature during deposition of the NbP and TaP, should be between about 900 and 1150 C., preferably about 1100" C. Niobium, in the form of dendritic crystals of niobium monophosphide, is first deposited at a potential of about 1.0 to 2.5 volts and a cathode current density of about 30 to 75 amp/dmF. The cathode is removed from the electrolyte and the NbP crystals are scraped off. The cathode is replaced and electrolysis continued at a potential of about 3.0 to 5.0 volts and a cathode current density of about to 200 amp/dm. to deposit tantatum, also in the form of phosphide. Time required for substantially complete deposition of each metal is about 1 to 2 hours.

The cell employed for the electrolysis is conventional and preferably consists of a graphite crucible serving as container and anode. The cathode is a centrally positioned graphite or refractory metal rod. The cell is desirably heated in an electric resistance furnace, but an oil or gas-fired furnace may also be used. A protective atmosphere is not required.

Niobium and tantalum metals may be readily recovered from their phosphides by means of conventional processes such as (1) dissolution in nitric or sulfuric acid to prepare oxides which are reduced to metal by metallothermic reactions or (2) chlorination at high temperatures to yield chlorides which are reduced to metal by metallothermic reactions.

The invention will be more specifically illustrated by the following examples.

EXAMPLE 1 Weight Mole Electrolyte composition percent percent (Nb, T9205 4 1 NmPzOy 21 6 B20; 11 11 NaCl 56 68 NaF. 9 14 1 50 wt. percent each 01 Nb O; and Tarot.

Cell potential, cycle A: 2.5 volts Cell potential, cycle B: 4.0 volts Cell current, cycle A: 40-70 amps Cell current, cycle B: 110-140 amps Addition between cycles A and B: 210 g. Na AlF and 30 g. K2C03 Duration of electrolysis and results are given in Table 1.

TABLE 1 Analysis of Recovery, Electrolysis Product product percent duration, weight, Cycle amp-hrs. gms. NbP TaP Nb Ta A 75 19.4 94.0 5.6 98 7 B 168 8.0 7.9 91.0 2 as Total 100 45 EXAMPLE 2 In this example, a columbite mineral concentrate was used as feed material. The following operating data and electrolyte composition were employed:

Cell configuration:

Anode: 3'' ID. x 7" high graphite crucible Cathode: 1" diameter graphite rod Electrode spacing: Cathode 1" from side walls and 1%" from cell bottom Operating temperature: 1,1 i10 C. Electrolyte weight: 1,000 grams Cell feed: Congo columbite concentrate Analysis, wt. pct.:

Nb O 52 T3205 F6203 MnO Weight Mole Electrolyte composition percent percent Cell potential, cycle A: 2.5 volts Cell potential, cycle B: 4.0 volts Cell current, cycle A: 55-70 ampercs Cell current, cycle '13: 120-130 amperes '4 Electrolysis duration, cycle A: 2 hours Electrolysis duration, cycle B: 1 hour Addition between cycles A and B: 200 g. Na AlF and 20 g. K 'CO Results are given in Table 2.

1. A method for recovery of niobium and tantalum, in the form of phosphide, from crude oxide or mineral feed material comprising dissolving the feed material in a molten electrolyte comprising an alkali metal phosphate, an alkali metal halide and an alkali metal borate or boric oxide, electrolyzing the molten mixture at a potential of about 1.0 to 2.5 volts to selectively deposit niobium phosphide at the cathode, removing the deposited niobium phosphide from the cathode, and subsequently electrolyzing the molten mixture at a potential of about 3.0 to 5.0 volts to deposit tantalum phosphide at the cathode.

2. The method of claim 1 in which the electrolyte consists essentially of sodium pyrophosphate, sodium chloride, sodium fluoride and boric oxide.

3. The method of claim 1 in which a mixture of cryolite and potassium carbonate are added to the molten electrolyte after deposition of the niobium phosphide and prior to deposition of the tantalum phosphide.

4. The method of claim 1 in which the niobium phosphide is deposited at a potential of about 2.5 volts.

5. The method of claim 1 in which the tantalum phosphide is deposited at a potential of about 4.0 volts.

References Cited UNITED STATES PATENTS 3,498,894 3/ 1970 Cuomo et al. 204-61 TUNG TA-HSUNG, Primary Examiner D. R. VALENTINE, Assistant Examiner 

